The present invention relates to electric power generators and, more particularly, to a stator having reduced forces transmitted by a dovetail connection between a stator core and a stator frame and a method for forming the same.
A stator of an electric power generator generally includes a stator core and a stator frame. The stator core typically has a cylindrical shape and consists of a stack of insulated steel laminations. Each lamination is a segment of a full circle that has slots on its inner diameter to accommodate a stator winding and dovetail slots on its outer diameter.
The stator frame has plurality of key bars connected thereto, each key bar having a dovetail to provide structural support for the stator core. During the assembly of the stator core, each lamination is installed into the stator frame by engaging a dovetail slot onto a corresponding key bar dovetail. Sufficient clearance must be provided between the key bar dovetails and the faces of the lamination which form the corresponding dovetail slots to allow for the easy assembly of the stator core considering the location and dimensional tolerances of the key bar and laminations. The clearance between the three faces of the key bar dovetail and the opposing faces of the corresponding lamination forming the dovetail slots varies randomly due to the random location of the key bar dovetail within its location tolerance along the length of the key bar. Due to this variable clearance, the actual contact locations between the key bar dovetail and the corresponding lamination (and hence the contact locations between the stator frame and the stator core) is randomly distributed.
When force loads are applied between the stator core and the stator frame, the loads are transmitted through the randomly distributed contact points. The magnitude of the force of a load at each contact point is a function of the stiffness at that contact point. If the stiffness is high at a particular contact point, then the magnitude of the forces at that point are high.
It would thus be beneficial to control the location of the contact points between the stator core and the stator frame. Specifically, it would be beneficial to control the location of the contact points between the key bar dovetails connected to the stator frame and the corresponding laminations which include the respective dovetail slots into which each dovetail is engaged. By controlling the locations of the contact points so that they are arranged at a known locations (and hence known stiffness), the interface force transmitted between the stator core and the key bar (and hence stator frame) can be evenly distributed and the maximum value of this interface force can be reduced. By reducing the maximum value of the interface force, the reliability of the connections can be increased.
In an exemplary embodiment of the invention, a method of forming a stator comprises providing a stator frame having a frame plate, connecting a plurality of key bars to the frame plate at respective connection points, each of the key bars having a dovetail, and providing a stator core comprising a plurality of laminations each having a dovetail slot formed therein. Each dovetail is engaged into a respective dovetail slot so that at least some of the dovetails contact respective laminations at respective contact points. The respective locations of the contact points are controlled such that a force load transmitted by the contact points is evenly distributed among the contact points. The locations of the contact points are also controlled such that the key bar stress at the connection points between the key bars and the frame plate is minimized and a stiffness of all of the contact points is equal. The stiffness is controlled by varying a distance between the contact points and the frame plate. The stiffness can be reduced by arranging the location of a contact point further away from the frame plate.
The locations of the contact points are controlled by varying a cross-sectional area of the dovetail slots such that at least two of the laminations respectively have dovetail slots with different cross-sectional areas. The locations of the contact points can be controlled by increasing the size of the dovetail slots in those laminations where a contact point is not desired and decreasing the size of the dovetail slots in those laminations where a contact point is desired. Alternatively, the locations of contact points can be controlled by respectively arranging wedges within some of the dovetail slots to form contact points between the dovetail and the respective laminations.
In another exemplary embodiment of the present invention, a stator comprises a stator frame having a frame plate, a plurality of key bars connected to the frame plate at respective connection points, each of the key bars having a dovetail, and a stator core comprising a plurality of laminations each having a dovetail slot formed therein. Each of the dovetails engages into respective dovetail slots so that at least some of the dovetails contact respective laminations at respective contact points, the contact points being located such that a force load transmitted by the contact points is evenly distributed among the contact points.
The contact points have locations such that key bar stress at the connection points is minimized and a stiffness of all the contact points is equal. The respective cross-sectional areas of the dovetail slots vary such that at least two of the laminations respectively have dovetail slots which have different cross-sectional areas. The cross-sectional areas of the dovetail slots in those laminations where contact points are not desired is larger than respective cross-sectional areas of the dovetail slots in those laminations where contact points are desired. Alternatively, the stator can further comprise wedges which are respectively arranged within some of the dovetail slots to form contact points between the dovetail and the respective lamination.